Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/Nadeko.Medusa/Attributes/prioAttribute.cs

@@ -0,0 +1,20 @@
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Sets the priority of a command in case there are multiple commands with the same name but different parameters.
+/// Higher value means higher priority.
+/// </summary>
+[AttributeUsage(AttributeTargets.Method)]
+public class prioAttribute : Attribute
+{
+    public int Priority { get; }
+
+    /// <summary>
+    /// Snek command priority
+    /// </summary>
+    /// <param name="priority">Priority value. The higher the value, the higher the priority</param>
+    public prioAttribute(int priority)
+    {
+        Priority = priority;
+    }
+}